1. Field of the Invention
The invention relates to a luminescent device using an organic luminescent element having an anode, a cathode, and a film (referred below to as “organic compound layer”), which includes an organic compound adapted to effect luminescence upon application of an electric field. More particularly, the invention relates to a luminescent device using an organic luminescent device whose organic compound film contains a high-molecular compound and which requires a lower drive voltage and has a longer life than luminescent devices of the related art, and a method of manufacturing this luminescent devices. Note that the luminescent device described in the specification of the present application indicates an image display device or a luminescent device, which use an organic luminescent element as luminescent element. Also, the luminescent device includes all of modules, in which a connector, for example, an anisotropic electroconductive film (FPC: Flexible printed circuit) or a TAB (Tape Automated Bonding) tape or a TCP (Tape Carrier Package) is mounted to an organic luminescent element, modules, in which a printed-circuit board is provided on a TAB tape or a tip end of a TCP, or modules, in which an IC (integrated circuit) is directly mounted on an organic luminescent element in the COG (Chip On Glass) system.
2. Description of the Related Art
An organic luminescent element is one adapted to effect luminescence upon application of an electric field. A mechanism for luminescence has been said to reside in that an organic compound layer is interposed between electrodes, upon application of voltage thereto electrons filled from a cathode and holes filled from an anode recombine together at a center of luminescence in the organic compound layer to. form excited molecules (referred below to as “molecule exciton”), and the molecule excitons discharge energy to produce luminescence when returned to the base state.
In addition, kinds of molecule excitons formed by the organic compound can include a singlet excited state and a triplet excited state, while the specification of the present invention contains the case where either of the excited states contributes to luminescence.
In such organic luminescent element, an organic compound layer is normally formed in a thin film below 1 μm. Also, since the organic luminescent element is a self-luminescent type one, in which the organic compound layer itself emits light, a backlight used in a conventional liquid crystal display is not necessary. Accordingly, the organic luminescent element can be very advantageously formed to be thin and lightweight.
Also, with, for example, an organic compound layer of about 100 to 200 nm in thickness, a time period having elapsed from filling of a carrier to recombination thereof is in the order of several tens of nanosecond taking account of the extent of movement of the carrier in the organic compound layer, and luminescence is achieved in the order of less than one micro second even when the procedure from the recombination of the carrier to luminescence is included. Accordingly, one of the features is that the speed of response is very large.
Further, since the organic luminescent element is a carrier-filling type luminescent element, it can be driven by DC voltage, and is hard to generate noise. With respect to drive voltage, an adequate luminance of 100 cd/m2 is achieved at 5.5 V by first making the thickness of an organic compound layer a uniform, super-thin film of around 100 nm, selecting an electrode material, which reduces a carrier filling barrier relative to the organic compound layer, and further introducing a single hetero structure (double structure) (Literature 1: C. W. Tang and S. A. VanSlyke, “Organic electroluminescent diodes”, Applied Physics Letters, vol. 51, No. 12, 913-915 (1987)).
Owing to such performances as thin and lightweight, high-speed responsibility, DC low voltage drive, and the like, organic luminescent elements have been given attention as next-generation flat panel display elements. Also, since organic luminescent elements are of self-luminescent type and large in angle of visibility, they are comparatively favorable in visibility and believed to be effective as elements used for displays in portable equipments.
Hereupon, in the constitution of an organic luminescent element described in Literature 1, a carrier filling barrier is made small by using as a cathode a relatively stable Mg:Ag alloy of low work function to enhance an electron injecting quality. This makes it possible to fill a large amount of carrier into the organic compound layer.
Further, the recombination efficiency of the carrier is improved by leaps and bounds by application of a single hetero structure, in which a hole transporting layer composed of a diamine compound and an electron transporting luminescent layer composed of tris (8-quinolinolato) aluminium (hereinafter written as “Alq3”) are laminated as an organic compound layer, which is explained below.
In the case of, for example, an organic luminescent element having only a single Alq3 layer, a major part of electrons filled from a cathode reaches an anode without recombining with holes, making the luminescent efficiency very low, since Alq3 is of electron transporting quality. That is, in order to have the single-layered organic luminescent element efficiently emitting light (or driving at low voltage), it is necessary to use a material (referred below to as “bipolar material”) capable of carrying both electrons and holes in well-balanced manner, and Alq3 does not meet such requirement.
However, application of the single hetero structure described in Literature 1 causes electrons filled from a cathode to be blocked by an interface between the hole transporting layer and the electron transporting luminescent layer to be enclosed in the electron transporting luminescent layer. Accordingly, the carrier is efficiently recombined in the electron transporting luminescent layer to provide for efficient luminescence.
When the concept of such carrier blocking function is developed, it becomes possible to control a carrier recombining region. As an example, there is a report, according to which success is achieved in enclosing holes in a hole transporting layer and making the hole transporting layer luminescent by inserting a layer (hole blocking layer), which is capable of blocking holes, between the hole transporting layer and an electron transporting layer (Literature 2: Yasunori KIJIMA, Nobutoshi ASAI and Shin-ichiro TAMURA, “A Blue Organic Light Emitting Diode”, Japanese Journal of Applied Physics, Vol. 38, 5274-5277 (1999)).
Also, it can be said that the organic luminescent element described in Literature 1 is based on, so to speak, that thought of functional separation, according to which carrying of holes is performed by the hole transporting layer and carrying and luminescence of electrons are performed by the electron transporting luminescent layer. Such concept of functional separation has further grown to a concept of double heterostructure (three-layered structure), according to which a luminescent layer is inserted between the hole transporting layer and the electron transporting layer (Literature 3: Chihaya ADACHI, Shizuo TOKITO, Tetsuo TSUTSUI and Shogo SAITO, “Electroluminescence in Organic Films with Three-Layered Structure”, Japanese Journal of Applied Physics, Vol. 27, No. 2, L269-L271 (1988)).
Such functional separation has an advantage in that the functional separation makes it unnecessary for a kind of organic material to have a variety of functions (luminescence, carrier carrying quality, filling quality of carrier from electrode, and so on) at a time, which provides a wide freedom in molecular design or the like (for example, it is unnecessary to unreasonably search for bipolar materials). That is, a high luminous efficiency can be easily attained by combining materials having a good luminous quality and a carrier carrying quality, respectively.
Owing to these advantages, the concept of the laminated structure (carrier blocking function or functional separation) itself described in Literature 1 has been widely utilized till now.
However, being a junction between substances of different kinds (in particular, a junction between insulating materials), the laminated structure described above will necessarily produce an energy barrier at an interface the substances. Since the presence of an energy barrier inhibits movements of a carrier at the interface, the two following problems are caused.
One of the problems is that it results in a barrier leading to further reduction of drive voltage. Actually, it has been reported with respect to existing organic luminescent elements that an element of a single-layered structure making use of a conjugate polymer is excellent in terms of drive voltage and holds top data (comparison in luminescence from the singlet excited state) in power efficiency (unit:“lm/W”) (Literature 4: Tetsuo Tsutsui “bulletin of organic molecular/bioelectronics” subcommittee of Society of Applied Physics, Vol. 11, No. 1, P. 8 (2000)).
In addition, the conjugate polymer described in Literature 4 is a bipolar material, and can attain a level equivalent to that of the laminated structure with respect to the recombination efficiency of a carrier. Accordingly, it demonstrates that a single layer structure having less interfaces is actually low in drive voltage provided that a method making use of a bipolar material can make an equivalent recombination efficiency of a carrier without the use of any laminated structure.
For example, there is a method, in which a material for mitigating an energy barrier is inserted at an interface between an electrode and an organic compound layer to enhance a carrier filling quality to reduce drive voltage (Literature 5: Takeo Wakimoto, Yoshinori Fukuda, Kenichi Nagayama, Akira Yokoi, Hitoshi Nakada, and Masami Tsuchida, “Organic EL Cells Using Alkaline Metal Compounds as Electron Injection Materials”, IEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 44, NO. 8, 1245-1248 (1977)). In Literature 5, the use of Li2O as an electron injecting layer has been successful in reduction of drive voltage.
However, the carrier transfer between organic materials (e.g., between the hole transport layer and luminescent layer; the interface will hereinafter be called “organic interface”) remains as an unsettled issue and is considered to be an important point in catching up with the low drive voltage provided by the single-layered structure.
Further, the other problem caused by an energy barrier is believed to be an influence on the service life of organic luminescent elements. That is, movements of a carrier are impeded, and brilliance is lowered due to build-up of charges.
While any definite theory has not been established with respect to such mechanism of deterioration, there is a report that lowering of brilliance can be suppressed by inserting a hole injecting layer between an anode and a hole transporting layer and employing not DC driving but AC driving of rectangular wave (Literature 6: S. A. VanSlyke, C. H. Chen, and C. W. Tang, “Organic electroluminescent devices with improved stability”, Applied Physics Letters, Vol. 69, No. 15, 2160-2162 (1996)). This can be said to present an experimental evidence that lowering of brilliance can be suppressed by eliminating accumulation of charges due to insertion of a hole injecting layer and AC driving.
It can be said from the above that on one hand the laminated structure has an advantage in enabling easily enhancing the recombination efficiency of a carrier and enlarging a range of material selection in terms of functional separation and on the other hand formation of many organic interfaces impedes movements of a carrier and has an influence on lowering of drive voltage and brilliance.